I was giving a presentation for my degree capstone. I was scheduled for the last block for the day, so I had a solid eight hours or so to listen and think about my own presentation.
By lunchtime, I decided that my prepared speech wasn't what I wanted to say, and decided to go off script. Granted, this is something I'd spent an absurd amount of time on, and I knew it really well. The audience was only people I'd worked with over the past couple of years and their family members (for me, my younger sister was in the room), so stage fright wasn't a possible issue, and I knew if I just got up there and started talking, I could make this what I wanted it to be.
My subject involved post-apocalyptic fiction. I walked up to the front of the room, started my presentation, and tossed my notes to the far corner of the table. I put both hands down on the podium, leaned forward, and said, "My name is Crimsonial, and I'm here to talk to you about the end of the world."
It got a good laugh, and the improvised speech was a really comfortable experience. It was a subject that I was excited about, and having workshopped with the majority of the room on it, I knew they were interested too. I knew what to say.